This invention relates to nonwoven fibrous structures and more particularly to breathable fabrics and sheet structures formed by fibers which are held together without weaving or knitting.
Nonwoven fibrous structures have existed for many years and today there are a number of different nonwoven technologies in commercial use. To illustrate the breadth of nonwoven technologies, paper is probably one of the earliest developed nonwoven fibrous structures. Nonwoven technologies continue to be developed by those seeking new applications and competitive advantages. One broad market area that has proven to be highly desirable because of its large volume and economics is the protective apparel market. This market comprises protection from hazardous chemicals such as in chemical spill clean up, from liquids such as blood in the medical field and from dry particulates or other hazards such as painting or asbestos removal.
It is known that for a garment to be comfortable, it must accommodate the body's physiological need for thermal regulation. In warm environments, heat energy must be expelled from the body. This is done principally by a combination of direct thermal conduction of heat away from the body through the fabric and air layers at the skin surface, convection of heat away from the body by flowing air, and by the cooling effects of evaporation of sweat from the surface of the skin. Clothing which appreciably inhibits heat transfer can cause heat and moisture buildup and this can result in discomfort due to warm, sticky, clammy and or sweaty sensations. In the extreme case, for example, where protective clothing prevents adequate thermal regulation during activity in a warm and humid environment, such clothing limitations not only lead to discomfort, but can result in life-threatening heat stress. For this reason, frequently, clothing limitations impose limitations on activity to avoid the consequences of heat stress.
Studies have shown that the most comfortable garments with the least restrictions on physical activity in warm, humid environments, are those most able to breathe through mechanisms of air exchange with the environment. (Bernard, T. E., N. W. Gonzales, N. L. Carroll, M. A. Bryner and J. P. Zeigler. “Sustained work rate for five clothing ensembles and the relationship to air permeability and moisture vapor transmission rate.” American Industrial Hygiene Conference, Toronto, June 1999; N. W. Gonzales, “Maximum Sustainable Work for Five Protective Clothing Ensembles and the Effects of Moisture Vapor Transmission Rates and Air Permeability” Master's Thesis, College of Public Health, University of South Florida, December 1998).
Physical activity flexes fabric and garment. If a fabric has low enough resistance to air flow, this, in turn, produces a pumping action which pushes and pulls air back and forth through the fabric. By this mechanism, the exchange of warm moisture laden air within the garment with ambient air provides a significant cooling effect. Tests of protective garments made of the same cut, but with widely differing air flow resistance under warm humid conditions (32° C., 60% RH), have shown that the garments made of fabrics with the least air flow resistance repeatedly allowed subjects to achieve higher levels of activity without incurring heat stress. Conversely, garments made of fabrics with the highest air flow resistance limited the physical activity of the same subjects to the lowest levels to avoid heat stress. Garments made of fabrics having intermediate air flow resistance allowed subjects to achieve intermediate levels of activity without heat stress. The intermediate activity levels correlated very well with the air flow resistance of the fabric.
Clearly, under conditions where the body must transfer heat and moisture to maintain comfort or avoid heat stress, it is desirable to for garments to be made with fabrics having low air flow resistance.
Clothing provides protection from hazards in the environment. The degree of protection clothing imparts is dependent upon the effectiveness of the barrier characteristics of the clothing. Where the function of the barrier is to keep environmental particulates or fluids from penetrating a garment to reach the wearer, barrier is easily correlated with fabric pore size. The most effective barriers generally have the smallest pore size.
Unfortunately, smaller pore size also generally results in higher air flow resistance. In the studies cited above, the garments with the highest barrier properties had the lowest airflow permeability and vise versa. So the ability to provide effective barrier protection in clothing and the ability to provide low air flow resistance, i.e., high air flow permeability, in the same garment are inversely related.
Hydrostatic head or “hydrohead” (AATCC TM 127-194) is a convenient measure of the ability of a fabric to prevent water penetration. It is presented as the pressure, in centimeters of water column (cmwc) required to force liquid water through a hydrophobic fabric. It is known that hydrohead depends inversely on pore size. Lower pore size produces higher hydrohead and higher pore size produces lower hydrohead.
Fabric air flow permeability is commonly measured using the Frazier measurement (ASTM D737). In this measurement, a pressure difference of 124.5 N/m2 (0.5 inches of water column) is applied to a suitably clamped fabric sample and the resultant air flow rate is measured as Frazier permeability or more simply as “Frazier”. Herein, Frazier permeability is reported in units of m3/m2-min. High Frazier, corresponds to high air flow permeability and low air flow resistance while low Frazier corresponds to low air flow permeability and high air flow resistance.
Microporous films have been used in barrier materials to achieve extremely high hydrostatic head liquid barrier properties, but at the expense of breathability, such that their Frazier permeabilities are unacceptably low, rendering fabrics containing such films uncomfortable for the wearer.
Currently, most melt-spun fibers have diameters on the order of several tens of micrometers, whereas melt-blown fibers are known to have fiber diameters on the order of from about 1 to 10 micrometers. Recently, many researchers have made efforts to decrease fiber sizes in order to obtain different benefits, as compared to conventional fibers.
Advances have been made in providing both high hydrohead properties and high Frazier properties in the same fabric. For example, U.S. Pat. No. 5,885,909 discloses low or sub-denier nonwoven fibrous structures which demonstrate an unusual combination of high Frazier permeability and high hydrostatic head liquid barrier properties.
More recently, efforts have centered around obtaining fiber diameters in the ‘nanofiber’ range, i.e. with diameters on the order of less than about 0.5 micrometers (500 nm). However, production of such small fibers has presented many problems including low throughput, poor efficiency in spinning and difficulties in fiber collection.
Conventionally, nanofibers have been produced by the technique of electrospinning, as described in “Electrostatic Spinning of Acrylic Microfibers”, P. K. Baumgarten, Journal of Colloid and Interface Science, Vol. 36, No. 1, May, 1971. According to the electrospinning process, an electric potential is applied to a drop of a polymer in solution hanging from a metal tube, for example a syringe needle, which results in elongation of the drop of the solution to form very fine fibers which are directed to a grounded collector. Fibers with diameters in the range of 0.05 to 1.1 micrometers (50 to 1100 nm) are reported. An example of a suitable electrospinning apparatus for forming the nanofiber-containing fabrics of the present invention is disclosed in U.S. Pat. No. 4,127,706, incorporated herein by reference.
The vast majority of investigations into nanofiber production reported in the prior art literature have been directed to formation of essentially hydrophilic polymer nanofibers, such as polyamide, polyurethane and the like. While some investigators have suggested that nanofibers could be produced from hydrophobic polymers, few actual examples of such hydrophobic nanofibers are disclosed in the literature. U.S. Pat. No. 4,127,706 discloses production of porous fluoropolymer fibrous sheet, suggesting the production of PTFE fibers with diameters in the range of 0.1 to 10 micrometers, but exemplifying only fibers with diameters of 0.5 micrometer and above.